1. Field of the Invention
The present invention relates generally to the field of imaging using photons and more specifically to imaging using coincidence photons generated from electron-positron annihilation.
2. Description of the Related Art
In positron emission tomography (“PET”) imaging, a radioisotope source emits positrons that combine with electrons and annihilate, each annihilation producing a pair of oppositely directed 511 keV photons. The volume being imaged is placed between a pair of position-sensitive gamma photon detector arrays. The coincident detection of a pair of photons on the two detector arrays signals that an annihilation event has been detected. The position of the annihilation event can be deduced as having occurred at some point along a straight line joining the two points of detection. Time-of-flight information can be used to somewhat further localize the position of the annihilation to some section along this line, limited by the timing resolution of the photon detection systems. The typical application is to use these projected points of annihilation to build up a three-dimensional map of annihilation density and measure the uptake of short-lived radioisotopes within human patients for diagnostic purposes.
PET imaging systems are designed to localize the position of the radioisotope materials. The invention described in the Detailed Description of the Exemplary Embodiments that follows modifies the PET imaging process so that annihilation coincidence photons can be used for imaging of other targets.
A landmine detection system has been described by J. R. Tickner, M. P. Currie, and G. J. Roach in “Feasibility study for a low-cost 3D gamma-ray camera,” Applied Radiation and Isotopes 61 (2004)67-71. This system uses a positron annihilation source to create probe photons with known directions and time. Instead of the use of a return-scatter directional detection technique, time-of-flight is used to establish some three-dimensional information as to the scattering locations. However, the prior art in time-of-flight scattering measurements using annihilation coincidence photons lacks accurate resolution in the third dimension through limitations in the state-of-the-art in radiation detector timing resolution.
So-called “flying-spot” backscatter imagers utilize a rotating collimator and a bremsstrahlung x-ray source to generate a rastered x-ray beam. When this beam is swept over a target, either by moving the source or the target through the rastering beam, a two-dimensional image is formed by detection of photons backscattered from the target. The flying-spot systems utilizing a bremsstrahlung x-ray source do not image in the third dimension. Also, because the outgoing probe photons from the x-ray source are not mono-energetic, it is not possible to use energy of the return scattered photons to discriminate single- from multiple-scatter events. Furthermore, bremsstrahlung x-ray sources produce many x-ray photons at lower energy that have significantly inferior penetration capability than higher-energy mono-energetic photons, and consequently produce images with less penetration and reduced contrast for a given fixed radiation dose to the target than a mono-energetic source will yield. X-ray equipment also tends to be complex and requires significant maintenance.